The present application claims priority and relates to Japanese Patent Application No. 2001-079889 filed on Mar. 21, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical scanning device having a light source which modulates a light flux by a pulse modulation method and an image forming apparatus including the optical scanning device.
2. Discussion of the Background
Laser printers and digital copying machines in recent years have progressed by increasing the resolution of an output image. Light sources of optical scanning devices for use for example in laser printers and digital copying machines have demanded that both the diameter of a stationary beam spot formed by a light flux on a scanned surface and the diameter of a scanning beam spot on the scanned surface be made smaller as the resolution increases.
Semiconductor lasers having an oscillating wavelength in a near-infrared region, for example about 780 nm, have been used mainly in the afore-mentioned optical scanning devices. However, a semiconductor laser having an oscillating wavelength in a near-infrared region has difficulty in producing a relatively small diameter scanning beam spot on a scanned surface because of diffraction limitations.
Assuming that an optical system for optical scanning devices does not have geometric aberrations, a lower limit value for a beam spot diameter (D) of a scanning beam spot on a scanned surface is expressed by the following formula: D=1.22xcex/NA, where xcex represents a wavelength of a light source and NA represents a numerical aperture of the optical system. That is, the beam spot diameter D of a scanning beam spot on a scanned surface is in direct proportion to the wavelength xcex of a light source. Accordingly, making the wavelength of a light source relatively small is effective for making the diameter of a scanning beam spot on a scanned surface smaller.
However, if the diameter of a scanning beam spot on a scanned surface of a photoconductor is made excessively small, a reciprocity phenomenon as discussed below occurs which broadens an electrostatic latent image formed by the scanning beam spot on the scanned surface, so that the resolution of an image resulting from the electrostatic latent image is decreased.
When a light source is configured to modulate light flux by a pulse modulation method, a scanning beam spot formed by the light flux on a scanned surface is more greatly affected by the pulse width of the light flux when the diameter of a stationary beam spot formed on the scanned surface by stationary light flux (hereinafter referred to as a stationary beam spot) is relatively small than when the diameter of the stationary beam spot is relatively large. A stationary beam spot formed by a light flux has stationary beam spot diameters substantially the same in the main scanning and sub-scanning directions. Because a scanning beam spot formed by the light flux on a scanned surface moves in the main scanning direction to scan the scanned surface, the diameter of the scanning beam spot on the scanned surface in the main scanning direction is larger than that in the sub-scanning direction. That is, when electrostatic latent images of vertical and horizontal lines having the same width are formed by a scanning beam spot which is relatively small, in the electrostatic latent images, the widths of the vertical and horizontal lines are different.
The present invention has been made in view of the above-discussed and other problems and addresses the above-discussed and other problems.
One object of the present invention is to provide a novel optical scanning device in which, even when a stationary beam spot diameter of a light source of the optical scanning device is made relatively small, the scanning beam spot diameters in the main scanning and sub-scanning directions are made substantially the same, and at the same time, the reciprocity phenomenon is prevented from occurring, so that broadening of an electrostatic latent image formed by the optical scanning device is avoided.
Further, another object of the present invention is to provide an image forming apparatus using an optical scanning device in which even when a stationary beam spot diameter of a light source of the optical scanning device is made relatively small, an electrostatic latent image of vertical and horizontal lines have substantially the same width when formed on a scanned surface of a photoconductor and the widths of the vertical and horizontal lines are made substantially the same so that an image having a relatively high resolution is outputted.
According to one preferred embodiment of the present invention, an optical scanning device for optically scanning a scanned surface of a photoconductor to form an electrostatic latent image thereupon includes a light source configured to emit a light flux and to modulate the light flux by a pulse modulation method. A deflector is configured to deflect the light flux from the light source. A scanning image formation element is included to condense the light flux deflected by the deflector to form a scanning beam spot on the scanned surface. The scanning beam spot scans the scanned surface and thereby forms the electrostatic latent image on the scanned surface. In the optical scanning device of this embodiment, the light source includes a semiconductor laser having an oscillating wavelength greater than 380 nm and smaller than 670 nm. A stationary beam spot, which is formed by the light flux on the scanned surface when the light flux is stationary, has a stationary beam spot diameter xcfx89m in a main scanning direction that is smaller than a stationary beam spot diameter xcfx89s in a sub-scanning direction. The scanning beam spot on the scanned surface moves substantially in the main scanning direction. The diameter xcfx89m and the diameter xcfx89s satisfy a following formula:
0.9xc3x97[(xcfx89m{circumflex over ( )}2+D{circumflex over ( )}2)xc3x97[{(xcfx89m+D)/xcfx89m}{circumflex over ( )}(xe2x85x9)]] less than xcfx89s less than 1.1xc3x97[(xcfx89m{circumflex over ( )}2+D{circumflex over ( )}2)xc3x97[{(xcfx89m+D)/xcfx89m}{circumflex over ( )}(xe2x85x9)]], 
where D=Doxc3x97Duty/100, Do is a size of a picture element, and Duty is a duty percentage in modulating the light flux at the light source.
In the above-described optical scanning device, the diameter xcfx89m and the diameter xcfx89s can be set such that broadening of the electrostatic latent image on the scanned surface due to a reciprocity phenomenon is prevented from occurring.
Further, in the above-described optical scanning device, the diameter xcfx89m and the diameter xcfx89s can be set such that a diameter of the scanning beam spot on the scanned surface in the main scanning direction and a diameter of the scanning beam spot on the scanned surface in the sub-scanning direction are substantially the same.
Furthermore, the above-described optical scanning device may include an aperture configured to shield a peripheral portion of the light flux emitted from the light source, and the diameter xcfx89m and the diameter xcfx89s can be regulated by widths of the aperture in the main scanning and sub-scanning directions.
According to another preferred embodiment of the present invention, an image forming apparatus includes a photoconductor and an optical scanning device configured as described above to scan a scanned surface of the photoconductor to form an electrostatic latent image thereupon.
Still another preferred embodiment of the present invention includes an optical scanning method for scanning a scanned surface of a photoconductor to form an electrostatic latent image thereupon. The method includes emitting a light flux and modulating the light flux by a pulse modulation method at a light source including a semiconductor laser having an oscillating wavelength greater than 380 nm and smaller than 670 nm, deflecting the light flux and condensing the deflected light flux to form a scanning beam spot on the scanned surface, and scanning the scanning beam spot on the scanned surface by substantially moving in the main scanning direction and thereby forming the electrostatic latent image on the scanned surface. In the optical scanning method of this embodiment, a stationary beam spot, which is formed by the light flux on the scanned surface when the light flux is stationary, has a stationary beam spot diameter Tm in a main scanning direction that is smaller than a stationary beam spot diameter xcfx89s in a sub-scanning direction, and the diameter xcfx89m and the diameter xcfx89s satisfy a following formula:
0.9xc3x97[(xcfx89m{circumflex over ( )}2+D{circumflex over ( )}2)xc3x97[{(xcfx89m+D)/xcfx89m}{circumflex over ( )}(xe2x85x9)]] less than xcfx89s less than 1.1xc3x97[(xcfx89m{circumflex over ( )}2+D{circumflex over ( )}2)xc3x97[{(xcfx89m+D)/xcfx89m}{circumflex over ( )}(xe2x85x9)]], 
where D=Doxc3x97Duty/100, Do is a size of a picture element, and Duty is a duty percentage in modulating the light flux at the light source.
In the above-described optical scanning method, the diameter xcfx89m and the diameter xcfx89s can be set such that broadening of the electrostatic latent image on the scanned surface due to a reciprocity phenomenon is prevented from occurring.
Further, in the above-described optical scanning method, the diameter xcfx89m and the diameter xcfx89s can be set such that a diameter of the scanning beam spot on the scanned surface in the main scanning direction and a diameter of the scanning beam spot on the scanned surface in the sub-scanning direction are substantially the same.
Furthermore, the optical scanning method may further include the step of shielding a peripheral portion of the light flux emitted from the light source, so that the diameter xcfx89m and the diameter xcfx89s are regulated by widths of the aperture in the main scanning and sub-scanning directions.